The purposes of the proposed work are to: 1. To determine the characteristics of the reaction of chloroacetaldehyde with heteroduplex DNA and RNA containing one base pair mismatch under conditions where GC and AT base pairs are of equal stability and C and A are equally reactive with chloroacetaldehyde. The mismatch containing heteroduplex will contain one strand with a conditional lethal mutation and one strand with a reversion. 2. To determine the efficiency of C1CH2CHO modified regions of DNA or RNA surrounding a mismatch to act as a specific substrate for endonuclease action. 3. To develop gel electrophoresis methods for radioiodinated DNA or RNA so that the location of a conditional lethal mutation may be physically mapped onto a viral chromosome using the methods above plus enzymatic or hybridization methods for assigning direction within a genome or genome fragment. 4. To apply the methods to physically mapping conditional lethal mutations of known base pair substitution in the genome of a double-stranded DNA bacteriophage. 5. To obtain a set of ts mutants of Sendai virus and to physically map these mutants on the genome.